[Grad2021] [Turnbull Zemi] Send me public keys! and more about "Using Git"

[Stephen J. Turnbull]

I've added a long section on a practical example (reducing version
clutter in your work folder), and a "toy" example making it clear how
git preserves your work even when you can't see it.  Direct URLs to
those sections:

https://turnbull.sk.tsukuba.ac.jp/Teach/Zemi/Workflow/using-git.html#adding-history-and-reducing-version-clutter

https://turnbull.sk.tsukuba.ac.jp/Teach/Zemi/Workflow/using-git.html#git-repo-as-hidden-folder-manager

I'm still waiting on user names (Latin letters and digits only, no
spaces please) and public keys from two students, and several students
who gave me keys don't seem to have pushed yet.  If you're having
trouble, please let me know *before* our meeting.